1. Field of the Invention
The present invention relates to hydraulic power systems with electrically operated control valves, and more particularly to electrical servomechanisms that control the application of electricity to such valves.
2. Description of the Related Art
A wide variety of machines have moveable members which are driven by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. For example, backhoes have a tractor on which is mounted a boom, arm and bucket assembly with each of those components being driven by one of more cylinder-piston arrangements. The flow of fluid to and from each hydraulic actuator is controlled by a hydraulic valve that traditionally has been manually operated by the machine operator.
There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing, as the control valves do not have to be located near an operator station, but can be located adjacent the hydraulic actuator being driven by the fluid. This change in technology also facilitates computerized control of the machine functions.
Application of pressurized fluid from a pump to the hydraulic actuator can be controlled by a set of proportional, solenoid operated pilot valves. These valves employ a solenoid coil which generates a magnetic field that moves an armature in one direction to open a valve. The armature acts on a pilot poppet which opens and closes a pilot passage that in turn causes a main valve poppet to move with respect to a primary valve seat located between the inlet and outlet of the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the solenoid coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another component is spring loaded to close the valve when electric current is removed from the solenoid coil.
When an operator desires to move a member on the machine, an input device, such as a joystick, is manipulated to produce an electrical signal that indicates the desired motion. This signal is received by a controller which responds by applying electric current to the solenoid valves connected to the hydraulic actuator associated with the machine member. To drive a cylinder-piston type hydraulic actuator, one solenoid valve is opened to supply pressurized fluid to a cylinder chamber on one side of the piston and another solenoid valve opens to drain fluid from the opposite cylinder chamber. Varying the magnitude of electric current applied to the solenoid valves alters the amount of fluid flow to the hydraulic actuator, thereby moving the machine member at proportionally different speeds.
Therefore, precise control of the electric current that is applied to the solenoid valve is essential for accurate control of the machine motion. However, it is difficult to precisely control the electric current. For one thing, the resistance of the solenoid coil changes significantly with temperature, which may vary from below −20° C. to over 100° C. for hydraulic equipment used outdoors. As a result, a given voltage level applied to the valve can produce different steady state electric current levels depending upon the temperature of the solenoid coil. Another factor affecting control accuracy is the back electromotive force (emf) that is generated as the solenoid armature moves. The back emf affects the net magnitude of electric current flowing through the solenoid coil. In addition, operation of the solenoid and the valve elements tend to be non-linear which makes their modeling difficult for control purposes.
Therefore, it is desirable to account for variation of the solenoid coil resistance when determining the magnitude of electric voltage to apply to open the hydraulic valve the desired amount.